There are numerous occasions where blending a fraction of smaller particles into larger particles are required. Often, when the smaller particles are very small, they tend to agglomerate among themselves, making it difficult to disperse them uniformly into the bulk of the relatively larger particulate materials. However, in many cases, a uniform dispersion is very important so that it is essential to break down as many agglomerates as possible and to break down the larger agglomerates of the smaller particles. One such example is in ultrafine powder coating where nano-sized particles need to be added into the fine coating powders to enhance their flowability and in some cases also to add special features to the coating surface.
It is generally known that powders become more difficult to handle as their sizes become smaller, because the strong interparticle forces make the particles agglomerate and the powder cohesive. The most important theories with regard to fine powder fluidization and handling are the Geldart Powder Classification (Geldart, D., Types of Gas Fluidization, Powder Technology, Vol. 7, 1973, 285-297) and the interparticle forces (Visser, J., An invited review—Van der Waals and other Cohesive Forces Affecting Powder Fluidization, Powder Technology, Vol. 58, 1989, 1-10). In 1973, Prof. Geldart of Bradford University in England proposed to divide all powders into four groups and used letters A-D to denote the groups as Aeratable, Bubbly-Ready, Cohesive, and Different (initially named Spoutable), respectively (FIG. 1). Geldart groups A and B powders are generally in the range of 25-35 microns to 700-900 microns. Both groups can be easily fluidized, but group A powders have a much higher potential to increase in volume when fluidized. Group D powders are about 700-900 microns to several millimeters in size and are more suitable for spouting than fluidizing. Group C powders are those smaller than about 25-30 microns and are very cohesive.
Typical characteristics of group C powders when subjected to fluidization include agglomerating and clumping into a packed bed, channeling or even completely defluidizing, and intermittency or choking when transported (Zhu, J., Fluidization of Fine Powders, Chapter 10 in Advances in Granular Materials: Fundamentals and Applications, London: Royal Society of Chemistry, 2003, pp 270-295). These features make group C powders very difficult to handle. When used for powder coating, these characteristics translate into poor fluidization in the feed hopper, uneven flow in the transport hose, sticking and accumulating on the inside of the hose and gun, and puffing at the spray gun, leading to an inconsistent coating surface that is unacceptable. This is the major hurdle that has prevented the powder coating industry from using group C fine powders. With a relatively lighter density, powder coatings usually have the Geldart group A-C boundary around 22-25 microns so that a powder coating with D50 in this range becomes very sensitive to its particle size distribution.
The cohesive nature of group C powder comes from the fact that when the particle size becomes smaller, the relative magnitude of the interparticle forces increases significantly in relation to the gravitational and drag forces exerted on the particles. Such strong interparticle forces make the individual particles cling to each other and therefore form agglomerates.
There are three types of interparticle forces, the van der Waals force, the electrostatic force, and the capillary force (Visser, J., An invited review—Van der Waals and other Cohesive Forces Affecting Powder Fluidization, Powder Technology, Vol. 58, 1989, 1-10; Zhu, J., Fluidization of Fine Powders, Chapter 10 in Advances in Granular Materials: Fundamentals and Applications, London: Royal Society of Chemistry, 2003, pp 270-295; and Seville, J. P. K., Willett C. D., and Knight, P. C., Interparticle Forces in Fluidization: a Review, Powder Technology, Vol. 113, 2000, 261-268).
Van der Waals force is a collective term taken to include the dipole/dipole, dipole/non-polar and non polar/non-polar forces arising between molecules (Seville, J. P. K., Willett C. D., and Knight, P. C., Interparticle Forces in Fluidization: a Review, Powder Technology, Vol. 113, 2000, 261-268). This force always exists and is usually the largest interparticle force among the three types. Van der Waals force only becomes noticeable when the particles come sufficiently close, for example 0.2 to 1.0 nanometer apart (Visser, J., An invited review—Van der Waals and other Cohesive Forces Affecting Powder Fluidization, Powder Technology, Vol. 58, 1989, 1-10). Van der Waals force may be understood by imagining molecules instantaneously possessing different electronic configurations, giving them a dipolar character. This temporary situation will act on the neighboring molecules, also rendering these dipolar. As a result thereof and as a consequence of the general attraction between dipoles, molecules attract each other, even when they are apolar singly.
Electrostatic force can occur by tribo-electric charging or by the formation of a potential difference when particles of different work functions are brought into contact. The resulting Coulomb attraction makes the powder adhesive. Capillary force comes from the fluid condensation in the gap between the particles in close contact, resulting in liquid bridging force among particles. It should be noted that a higher capillary force comes at the expense of the electrostatic force, which diminishes with the increase of moisture.
Due to the strong interparticle forces, Group C powders are generally considered non-fluidizable and non-transportable pneumatically, and therefore, non-sprayable. In term of fluidization, fluidization is sometimes possible but in the form of agglomerates instead of primary particles. As a result, Geldart C powders are considered unusable in most cases. On the other hand, demands for the use of group C powders are increasing in many industries because the smaller particle size does bring many benefits. For example, group C particles have been widely used in new advanced materials and chemical industries due to their special characteristics. With their high specific surface area, smaller primary particles commonly lead to a better quality of final product in the ceramic industry or in powder metallurgy. Finer powders also lead to significant improvement of the coating finish in the powder coating industry (Zhu J and Zhang H, Ultrafine powder coatings: An innovation, Powder Coating, 16(7), 39-47, 2005; and Zhu J and Zhang H, Fine powder coatings offer many other advantages besides better surface quality and thinner films, Powder Coating, Feb. 7, 2006.). Aerogel powders can provide very high surface area for catalytic chemical reactions. Moreover, fine and ultrafine powders are of increasingly important in the pharmaceutical, plastics, and food industries (Zhu, J., Wen, J., Ma, Y. and Zhang, H., 2004, Apparatus for Volumetric Metering of Small Quantity of Powder from Fluidized Beds, U.S. Pat. No. 6,684,917, and Zhu, J., Luo, Y., Ma, Y. and Zhang, H., Direct Coating Solid Dosage Forms Using Powdered Materials”, U.S. Patent, filed 2005). Therefore, solutions to these problems will break the barrier to the applications of fine paint powder and open up very promising markets for the powder coating industry and other industries.
In response to these needs, different measures have been taken to enhance the flowabilities, fluidization, and transportation of these group C powders. Those measures are usually referred to as fluidization aids, which include mechanical stirring, acoustic, mechanical, or ultrasonic vibrating, addition of much larger particles to provide extra stirring, and pulsation of fluidization-gas, just to mention a few. Some of these measures are more effective than others for a given group C powder, but the effectiveness of almost all of these measures tends to diminish as the powder becomes finer in size. Adding much smaller particles such as fumed silica particles is another way to increase the fluidization capability of some group C powders and has been practiced in the industry for decades. On the other hand, adding many other finer particles has not ed increase the flowability of fine powders. Therefore, the mechanism is not yet clearly understood, although some have speculated that a “lubricant” effect may be occurring.
Powder coating is an environmentally friendly technology because it eliminates the use of organic or inorganic solvents and makes it possible to reuse the over-sprayed paint. However, the current technology with powder coatings does not provide as high a finish quality as “wet coatings”, hindering the further growth of the application of this technology. As mentioned in U.S. Pat. No. 5,171,613, powder coatings are generally characterized as having poor film uniformity, poor distinctness of image and with a heavy orange peel look. Also, excessive film thickness is required to obtain even such limited performance properties, because thinner films are difficult to obtain due to the large particle size.
Currently, many important coatings, such as color coats and clear coats on car/truck bodies, are still wet coated, due to the quality problems and excessive thickness associated with powder coatings. While powder coating has begun to be used as primers for automobile bodies, there are problems such as with particle agglomerations, causing imperfections to the coating surface such as seeds and particle balls, which require post polishing. Such post polishing, while trouble-some, is possible for primer but not realistic for top clear coat.
The lower quality surface finish of powder coating (“orange peel” imperfections etc.) and unnecessary excessive thickness, normally 45-50 microns and more, are mainly caused by the large average particle size of 30-50 microns currently used in the powder coating industry. It is understood that ultrafine powders with average particle size of <20 microns can greatly improve the quality of powder coating finishes, making them comparable with wet coating finishes. At the same time, the said ultrafine powders can also make it possible to apply thin film coatings of 10 to 25 microns or even less. However, coating powder of less than 20 microns fall into the Geldart C group and is therefore difficult to apply in the powder coating industry.
The powder coating industry has worked on improving the flowability of fine paint powder to make their applications possible, for example, U.S. Pat. Nos. 5,567,521, 5,498,479, 5,635,548, and 5,948,866. Some of them proposed to add fluidization additives (such as silica, aluminum oxide, aluminum hydroxide and other minerals etc.) to the fine powder to increase its fluidity. Of these patents, some of them also specified the methods of incorporating the additives into the fine powder (U.S. Pat. Nos. 5,567,521 and 5,498,479) while most others did not. These patents which did mention the incorporating method specified that the additives are dry-blended with the fine powder using a high-velocity (high-shear) mixer, such as Henschel Mixer available from Mitsui Miike Co., Ltd.
In addition to powder coating, there are also cases where mixing and dispersion of a small percentage of relatively finer particles into bulk larger particles are required and a well dispersed mixing at the micro level can be beneficial. For example, selective laser sintering is a process where metal particles coated with a thin layer of plastic materials (or some other materials which have a low melting point) are “fused” together by laser beams to form a structured part.
The function of the plastic coating is to “glue” the metal particles together and thereafter the plastic materials are completed removed during high temperature sintering where the metal particles are finally “fused” together. In this process, it is beneficial to use smaller metal particles so that the final part has smoother surface. In order to ensure the finer metal particles flow properly, superfine plastic (or other material) particles can be added. In this case, however, the addition of superfine plastic particles not only improves the flowability of the bulk metal particles, but can also form a layer of superfine plastic particles, if dispersed well, on each larger metal particle. Such a “particle coating” can replace the plastic coating applied currently, leading to significant savings by cutting out an extra step. It should be noted that this can also be expanded to the cases of superfine plastic particles coated on another type of plastic particles and superfine metal particles with lower melting temperature to larger bulk metal particles having high melting temperature. Additionally, such a coating layer does not need to be a continuous layer, to act as a “gluing” agent.
The inventors have been pursuing breakthroughs in the applications of fine powder technology, including fine paint powder coating technology. Some key technologies have been developed for uniform dispersion of fine and superfine particles into bulk particulate materials, for the smooth fluidization and pneumatic transportation, and for uniform spray of fine powders onto the product surfaces. One aspect of the technologies comprises the proposed fluidization additives and the formulation of these additives which provides the best fluidization aids to the fine powder (U.S. Pat. No. 6,833,185). In particular, U.S. Pat. No. 6,833,185 discloses that in order for the fine powder additives to work on the improvement of flowability of the fine powders, the additive particles need to be significantly smaller and also to have a much lower apparent particle density. Only when these two conditions are met are such additives effective in improving flowability. A higher ratio normally gives a better improvement to the flowability.
In various industries involving powder handling, higher powder flowability is generally needed for operations in powder manufacturing and application. For example, it has been practiced for decades that fluidization additives such as silica and aluminum oxide are dry blended into the powder to enhance its flowability and thus make the operations feasible. Fluidization additives are usually blended into the bulk powder in a blender (such as a tumbler or some other kind of low-shear mixer) or in some cases in a high shear mixer, after the powder is made. In some occasions, the fluidization additives are incorporated into the powder before the powder is mechanically pulverized.
For instance, some regular size powder coatings (D50>30 to 35 microns) exhibits strong cohesiveness because of the addition of one or more chemical component in the powder formulation, and therefore fluidization additives (silica and aluminum oxide) are commonly added into the paint chips to be ground. In this case, the additives are dispersed into the powder through the powder pulverizing process which resembles a high shear mixing process. For fine powders, the only mixing method proposed by other researchers is the use of a high shear mixer (U.S. Pat. No. 5,567,521, U.S. Pat. No. 5,498,479).
However, these blending methods suffer from several drawbacks. One such drawback is that the mixing temperature and time have to be tightly controlled for temperature sensitive materials (including paint powders and other powders comprising organic materials). In most cases, there always exist “dead regions” in the high-shear mixer, and therefore, non-dispersed additives are left in the powders which is detrimental for many cases. For example, in fine powder coating applications, the non-dispersed additives in powder coatings, mostly in agglomerated forms, would cause defects (seeds and bits) on the finished surfaces.
In addition, in the mixing methods where the additives are incorporated before the milling process, some of the additives (about 30%-80%) are known to be lost through the cyclone to the bag house (if a cyclone is employed to collect the milled powder). The remaining additives are also not necessarily uniformly dispersed, to act effectively as flow agent. During the mixing process, some of the additive particles could be “pushed into the particle surfaces of the powder being modified, leading to loss of functionality of the additives.
These drawbacks are affordable for some applications such as regular size powder coatings. Firstly, regular coating powders do not need as much fluidization additives as fine coating powders, which means that much less additive agglomerates will be present and thus much less chances of causing finish defects. Secondly, a regular size powder coating normally forms a coating film of 60-100 microns. A film with this thickness would hide almost all the additive agglomerates and show a defect-free finish.
For a fine powder, especially for a fine coating powder, however, some of these drawbacks would no longer be affordable. A representative film thickness of fine powder coating is around 10 to 50 microns. The inventors have discovered that the sizes of additive agglomerates resulting from the above-mentioned prior-art mixing methods (such as high-shear mixer) often reach 50 microns and occasionally reach 100 microns, depending on the mixing shear stress, mixing time and how inactive the “dead region” is in the high shear mixer. Many seeds and bits are normally present on the fine powder coating finishes, if the powder is prepared using these known mixing methods. This is due to the fact that some of the additive agglomerates are too large to be covered by the paint film.
Therefore, more effective mixing methods with following characteristics are highly desirable for fine powders. Such mixing methods should not only ensure homogenous mixing of fluidization additives into the fine powder in a macro-scale, but also do so on a micro-scale. Such a micro-level mixing step is needed for very good dispersion of additives into the fine powder, to ensure the maximum functionalities of the additives is realized. The mixing methods should ensure that the agglomerate sizes of the dry blended additives are smaller than the size of the bulk particles themselves. For powder coating, the agglomerate size should be smaller than the film thickness of the fine powder paint application so that no defects would be caused by the presence of any additive agglomerates. The mixing methods should retain, preferably, as much of the ultrafine additives in the fine powder as possible to maintain a lower cost.